Blender with Niftools addon, how can I export an fbx file with a texture linked to it?
This is likely because the texture is not set as a material for the object, but is still showing up in Texture Paint mode because it is in the tool’s active slot. Was the NIFtools addon originally designed for 2.79?
Anyway, you need to actually set up the texture as a material in Object or Edit mode. More on the subject can be found here: Blender Manual: Materials, Index